1. Field of the Invention
The present invention relates to portable devices and methods for performing in situ chemical analysis of aqueous environments, and, more particularly, to such devices and methods for performing mass spectrometry.
2. Description of Related Art
Mass spectrometry (MS) is known to be a versatile and powerful chemical sensing technique. In all known mass spectrometers analytes are transported from their normal state (e.g., solid phase or solution) into the vacuum of the MS through a sample interface. After entering the vacuum system, ionized analytes are then dispersed according to their mass-to-charge ratio (m/z) by some combination of electrical and magnetic fields. The ion signal is recorded as a function of m/z, typically using a high-gain electron multiplier or Faraday-cup detector. Measured intensities for each m/z result in the mass spectrum and can often be related to the concentration of the analyte in the original sample, or possibly be used for identification of unknowns in a complex mixture. Certain types of mass spectrometers allow multiple stages of mass spectrometry (K. L. Busch et al., Mass Spectrometry/Mass Spectrometry: Techniques and Applications of Tandem Mass Spectrometry, VCH, New York, 1988; C. Feigel, Spectroscopy 9, 31-40, 1994); two-stage analysis is denoted tandem mass spectrometry (MS/MS). Tandem mass spectrometry is typically accomplished by selecting ions of a particular m/z in the first stage of the MS and allowing them to collide with a gas target. The molecular fragments created in these energetic collisions are then analyzed according to their m/z in the second stage of the MS. The fragment mass spectrum can be used to deduce molecular structure and to provide more positive identification of chemicals in complex samples.
Although prior known mass spectrometers have been large laboratory instruments, smaller portable systems have become available, including those intended for use in harsh environments (C. M. Henry, Anal Chem. 71, 264-68A, 1999).
Remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) offer an attractive means for obtaining data in harsh underwater environments. These systems impose fairly stringent size and power constraints, with current devices limited to power supplied by 48 Vdc batteries for approximately 4 h, diameters less than 1 m, and lengths of approximately 2 m. An ROV-based submersible gas chromatograph-mass spectrometer (GCMS) system with automated membrane introduction was described in an article by G. Matz and G. Kibelka. The submersible GCMS system uses a large ion pump and is a significantly larger instrument than the portable instrument of the instant application, requiring a crane to lift, and having a shorter effective operation time in the field.
Some of the challenges faced in creating underwater mass spectrometry systems are related to the necessity of performing mass spectrometry in a vacuum (of the order of 10xe2x88x925 Torr). Analytes must be transported from the aqueous environment into a vacuum system, underwater. Since analysis of aqueous samples inevitably increases gas loads on vacuum pumps, use of entrainment or capture pumps would require frequent regeneration. Alternatively, if throughput pumps are used in a closed system, the inevitable increase in exhaust pressure of these pumps would eventually degrade pump operation. Since ambient underwater pressure increases by approximately 1 atm with 10-m depth increments, regeneration of entrainment pumps or decompression of pump housings becomes impractical at substantial depths.
There are additional challenges related to the desire to analyze these analytes, which may be present over a large range of concentrations (e.g., from 1 M for Na and Cl to 10xe2x88x9214 M for Au and Bi in the ocean) and in a variety of states (e.g., volatile, involatile, and complexed). For example, no single configuration of mass spectrometer is useful for analysis of this extremely wide range of compounds.
Thus there remains a need in the art for underwater mass spectrometer systems that is versatile, portable, and able to operate for a sustained period underfield conditions.
It is therefore an object of the present invention to provide an integrated mass spectrometer adapted for underwater operation.
It is an additional object to provide such a spectrometer that is autonomous.
It is a further object to provide such a spectrometer that is portable.
It is another object to provide such a spectrometer capable of performing mass-spectral analysis of a wide variety of chemical species.
It is yet an additional object to provide such a spectrometer adapted for detection of volatile analytes dissolved in a fluid.
These objects and others are achieved by the present invention, a portable mass spectrometer adapted for underwater use. The device comprises a watertight case having an inlet and means for transforming an analyte molecule from a solution phase into a gas phase positioned within the case. Means for directing a fluid to the transforming means from the inlet and means for analyzing the gas-phase analyte molecule to determine an identity thereof are also positioned within the case.
This system and method enable in situ underwater chemical analysis at a depth of at least 30 m with ppb detection limits for some volatile organic compounds (VOCs) and dissolved gases, such as those of interest to regulatory agencies and marine science. Alternative embodiments provide broader analytical access to chemical species in the water column. Future embodiments are planned, including networks of underwater vehicles capable of tracing chemicals, both natural and anthropogenic, to their sources (D. P. Fries et al., xe2x80x9cIn-Water Field analytical Technology: Underwater Mass Spectrometry, Mobile Robots, and Remote Intelligence for Wide and Local Area Chemical Profiling,xe2x80x9d Field Analytical Chemistry and Technology 5(3): 121-30, 2001).